1. Technical Field
The present disclosure relates to conductive paste, in particular, to the conductive aluminum paste applied in the electro-optic device.
2. Description of Related Art
In recent years, due to the shortage of energy and the importance of environmental protection realized by people, the demand of the pure energy for people is getting bigger and more urgent. Currently, pure energies being most potential comprises the wind power, the water power, the tidal energy, the geothermal power, the biological energy, the energy of the fuel cell, the solar energy, and so on. The most important one of the above pure energies is the solar energy. The solar energy has inexhaustible property, but does not pollute the environment of the earth, therefore being the mainstream of the pure energy.
The semiconductor substrate material of the silicon solar cell is the monocrystalline silicon or the polycrystalline silicon. In recent years, the compound semiconductor or the glass substrate material is developed to be the semiconductor substrate material of the silicon solar cell. The structure of the conventional silicon solar cell is illustrated as follows. A P-type diffused layer and an N-type diffused layer are sequentially formed on the semiconductor substrate, and the P/N junction is therefore formed between the P-type diffused layer and the N-type diffused layer. The energy of the incident light can separate the electron and the hole of the electron-hole pair in the P/N junction. The ohmic contacts (i.e. conductive metal sheets) are respectively formed on the P-type diffused layer and the N-type diffused layers, wherein the ohmic contacts are served as the electrodes of the silicon solar cell. The electrodes are used to conduct the electrons and the holes of the electron-hole pairs being separated to the external of the semiconductor substrate, and the process of the conversion which the light energy is converted to the electrical power is therefore finished.
Regarding the general fabrication of the silicon solar cell, the diffusion of the phosphorus atoms and the boron atoms are used to fabricate the N-type diffused layer and the P-type diffused layer, such that the P/N junction is formed. Moreover, the specific surface process is used to form an anti-reflection layer with the texture structure, such that the reflection of the incident light is reduced, and the strength of the incident light is also increased.
The electrodes of the conventional silicon solar cell are usually formed on the two sides of the semiconductor substrate by using the screen printing manner, the smearing manner, or the vacuum coating manner, and that is, the two electrodes are respectively a light incident side electrode and a back side electrode, wherein the light incident side electrode comprises a bus electrode and a finger electrode. The bus electrode and the finger electrode (i.e. the light incident electrode) are usually made of the silver paste. The area of incident light is increased to enhance the electro-optic conversion efficiency, and due to the requirement for increasing the area of incident light, the surface area of the light incident side electrode must be reduced. In other words, to increase the area of the incident light, the surface area of the light incident side electrode is designed to be small as possible. The back side electrode can be located on the entire back side of the semiconductor substrate since the light does not illuminate the back side (or the back side electrode) of the semiconductor substrate. Since the light does not illuminate the back side of the semiconductor substrate, the back side electrode is usually made of the silver-aluminum paste and the aluminum paste. After the light incident side electrode and the back side electrode are smearing on the two sides of the semiconductor substrate, the sintering process is then performed. In the sintering process, the high temperature (usually in the range from 600 degrees centigrade to 1000 degrees centigrade) is used to sinter the semiconductor substrate which the light incident side electrode and the back side electrode are smearing on the two sides, such that the aluminum atoms in the aluminum paste are diffusing into the P-type semiconductor (P-type diffused layer) to form the P+ layer. Therefore, the value of the open circuit voltage Voc in the circuit is increased, and the back side surface field (BSF) is also generated to enhance the electro-optical conversion efficiency of the silicon solar cell. It is noted that, the ohmic contact between each electrode and the semiconductor substrate must be maintained well, and the resistance value of the ohmic contact must also be maintained low, such that the electro-optical conversion efficiency of the silicon solar cell the electro-optical conversion efficiency of the silicon solar cell can be enhanced.
The electrode affects the stability of the electro-optical conversion efficiency of the silicon solar cell much. The usual standard value for evaluating the performance of the electrode is the fill factor (FF) of the silicon solar cell. If the resistance value of the serially connected resistors changes to be higher, the fill factor of the silicon solar cell will change to be smaller. The serially connected resistors are formed by the resistor of the ohmic contact between the P-type diffused layer and the electrode, the resistor between the P-type diffused layer and the electrode (i.e. ohmic contact), the resistor between the N-type diffused layer and the other one electrode, and the resistors of the electrodes.
Hence, to obtain the higher electro-optical conversion efficiency and the higher stability, some people are dedicated to the improvement of the fabrication material of the electrodes.